A building-permits office contrasts hop-by-hop IP routing with MPLS on its leased civic WAN. Why does MPLS label switching differ from pure IP routing at describe depth?
Select an answer to reveal the explanation.
Short Explanation
Pure IP is 'read the whole address book at every intersection.' MPLS is 'follow the sticker already on the package'—transit hops switch on labels instead of redoing longest-match IP each time.
Full Explanation
A building-permits office comparing hop-by-hop IP routing with MPLS should focus on how transit decisions are made. In pure IP routing, each router repeats a longest-match lookup on the destination IP. With MPLS label switching, once labels are imposed, transit LSRs forward based on the label in the LFIB rather than repeating that full IP longest-match lookup at every hop along the LSP. That is the describe-depth difference the exam targets: labeled switching in the core versus IP lookup at every node.
Requiring every P router to run a full Internet BGP table for civic CE prefixes is not how MPLS cores are designed. P routers typically forward on transport labels and avoid carrying customer or Internet BGP tables; PE routers handle BGP/VPN edge roles.
Claiming MPLS disables the IGP entirely so only static routes can build LSPs is wrong. Classic LDP LSPs follow IGP-learned reachability; the IGP remains foundational under LDP.
Stating that MPLS encrypts every packet by default, which IP routing cannot do without ACLs, confuses forwarding with confidentiality. MPLS does not encrypt by default; encryption needs a separate mechanism such as IPsec.
Exam caveat: edge PE routers still perform IP/VPN lookups at imposition/disposition points. The "no IP lookup" statement applies to transit label switching along the LSP, not to the entire network including CEs. Also distinguish LDP vs TE path selection.
Operational check: on a P router, show mpls forwarding-table should drive forwarding by in-label without needing CE prefixes in the global IP table; compare with CEF IP lookups on unlabeled edges. Confirm civic CE prefixes appear on PEs/VRFs, not necessarily on every P.